1. Technical Field
The present invention relates to an image processing device that decodes image data having been encoded by any predetermined encoding format, and at the same time, subjects the resulting decoded image data to resolution conversion to make the data of any predetermined image data size.
2. Related Art
When any decoded image data is not in a desired image data size resolution conversion is performed to make the decoded image data to be in the desired image data size, Such resolution conversion is performed in an image display device that decodes encoded image data by various types of formats such as JPEG (Joint Photographic Experts Group), and displays an image of the resulting decoded image data, and an image processing device such as printing device that prints the image of the decoded image data. In the below, this image data size is referred also to as “resolution”.
The function of such resolution conversion is generally implemented by, in an image processing device, decoding encoded image data in its entirety, temporarily storing the resulting decoded image data in its entirety into a memory, and reading the decoded image data stored in the memory for resolution conversion.
The function of such resolution conversion can be also implemented by, while decoding encoded image data of a JPEG format on the basis of an MCU (Minimum Coded Unit), subjecting the resulting image data to resolution conversion with a unit of an 8-by-8 pixel block configuring an MCU. Patent Document 1 (JP-A-2003-84738) also describes such an example.
The problem with the image processing device of such a general configuration as above is that the decoded image data is required to be entirely stored on a temporary basis in a memory, thereby needing a memory of a large capacity corresponding to the size of the decoded image data. With the entire decoded image data temporarily stored in the memory as such, resolution conversion is performed by reading the decoded image data stored in the memory. As a result, the larger size of the decoded image data, i.e., the higher resolution, increases the frequency of accessing the memory. This thus requires a longer time for memory access as such, and the longer time needed for such access prevents the increase of the processing speed in the entire image processing device. This results in the need for restricting the range of resolution available for conversion for an image represented by the decoded image data.
Another issue here is that, with the image processing device of Patent Document 1, resolution conversion is performed on the basis of a block. This indeed achieves resolution conversion of decoded image data with a small-capacity memory, and relaxes restrictions on the range of resolution available for conversion. However, this easily causes a noise (referred also to as “block noise”) that impairs the pixel continuity in an image at the border of the blocks, thereby possibly deteriorating the quality of the image after resolution conversion.